This invention relates to a novel method of crosslinking unsaturated block polymers. These polymers contain a tertiary carbon atom and may especially be polymers of conjugated diene monomers. Additionally, it relates to novel polymers made by the claimed method and adhesives and sealants made with the polymers.
It is known that a polymer can be obtained by an anionic polymerization of a conjugated diene compound or copolymerization with another diene or an alkenyl arene compound by using an organic alkali metal initiator. Thus, homopolymers and random and block copolymers can be made. Block copolymers have been produced which comprise primarily those having a general structure EQU A--B and A--B--A
wherein the polymer blocks A comprise thermoplastic polymer blocks of alkenyl arenes such as polystyrene, while block B is a polymer block of a conjugated diene. The proportion of the thermoplastic blocks to the elastomeric polymer block and the relative molecular weights of each of these blocks is balanced to obtain a rubber having unique performance characteristics. When the content of the alkenyl arene is small, the produced block copolymer is a so-called thermoplastic rubber. In such a rubber, the blocks A are thermodynamically incompatible with the blocks B resulting in a rubber consisting of two phases; a continuous elastomeric phase (blocks B) and a basically discontinuous hard, glass-like plastic phase (blocks A) called domains. Since the A--B--A block copolymers have two A blocks separated by a B block, domain formation results in effectively locking the B blocks and their inherent entanglements in place by the A blocks and forming a network structure.
These domains act as physical crosslinks anchoring the ends of many block copolymer chains. Such a phenomena allows the A--B--A rubber to behave like a conventionally vulcanized rubber in the unvulcanized state and is applicable for various uses. For example, these network forming polymers are applicable for uses such as moldings of shoe soles, impact modifiers for polystyrene resins and engineering thermoplastics, in adhesive and binder formulations and in the modification of asphalt.
Conversely, as the A--B block copolymers have only one A block, domain formation of the A blocks does not lock in the B blocks and their inherent entanglements. Moreover, when the alkenyl arene content is small resulting in a continuous elastomeric B phase, the strength of such polymers is derived primarily from the inherent entanglements of the various B blocks therein and to a lesser extent the inherent entanglements of the A blocks therein. Other non-network-forming polymers include homopolymers of conjugated dienes and copolymers of at least two conjugated dienes.
Both the network forming and non-network forming polymers are physically crosslinked. Light covalent crosslinking can be used to reinforce the physical crosslinking already present in such polymers and makes these polymers less susceptible to property losses at high temperature or in the presence of solvents and plasticizers. This allows them to be used in a broader array of applications such as high temperature masking tapes, permanent automotive tapes and sealants and permanent laminating adhesives.
Such reinforcing crosslinking can be achieved by radiation curing. However, radiation curing has certain disadvantages including the necessity for additional expensive equipment when EB processing or photoinitiation and acrylic monomers when using UV. If chemical crosslinking could be utilized, certain of these disadvantages could be eliminated. Amino resins have been widely used to crosslink various polymers and resins. However, up to the present time it has been thought that chemical crosslinking could only be achieved in polymers and resins which contained carboxyl groups or other functional groups. For instance, see 50 Years of Amino Coating Resins, edited and written by Albert J. Kirsch, published in 1986 by American Cyanamid Company, which describes in detail a whole series of amino resins which are useful in the present invention. It is stated therein on page 20 that the backbone polymers, i.e., the polymers which are to be crosslinked, "must contain one or more of the functional groups--hydroxy, carboxy, amide--listed above to be useful with amino resins". The foregoing publication is herein incorporated by reference. I have unexpectedly found that I can carry out crosslinking of the unfunctionalized unsaturated polymers described below using such chemical agents.